Electric eel ten times the power of a taser

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A large electric eel can deliver a shock ten times more powerful than a TASER, a new study has shown.

Electric eels are natives of the Amazon and have long fascinated scientists. Early electrician Michael Faraday worked with them, and 19th Century scientist Alexander von Humboldt documented a skirmish between electric eels and horses in the Amazon.

The animals operate essentially like an underwater battery. Specialised muscle cells called "electrocytes", which are distributed along the length of the eel's body, each pump charges across their membranes to accumulate a small voltage. Connected in series, these small voltages sum to a considerable potential: 500 volts is not unusual for a large specimen.

But how much current flows through a potential victim, and how these animals choose to deploy their built-in stun guns isn't well understood, so Vanderbilt University's Ken Catania set out to find out, using himself as a study subject and documenting his findings in a paper in the journal Current Biology.

One of the behaviours that Catania was eager to understand were Humbolt's descriptions of eels rearing up to attack horses. Catania had noticed that, when retrieving specimens with a conductive net, the eels would often rise up out of the water and attempt to stun the net itself. This appears to be an evolved example of offense being the best form of defense, perhaps driven by periodic, seasonal entrapment of the animals in shrinking pools where they might fall victim to crocodile or cat attack in the wild. Striking first to shock a potential predator might act as a suitably powerful warning to make them think twice.

But in order to explain why the eels rear up out of the water, and the likely effectiveness of this behaviour, it's necessary to know how much current is being communicated to the victim's body. For this, you need a relationship called Ohm's Law, which says that the current (I) is equal to the voltage divided by the resistance.

Measuring the voltage is relatively easy. Catania confirmed, using a pair of conductive gloves wired up to a voltmeter that the 40 centimetre juvenile he was studying registered about 200 volts. The resistance is more tricky. The resistance of water can be tested directly, and the resistance of the eel's own body could be measured by getting the animal to strike at a metal plate connected to the water and measuring the voltage when the eel reared to different heights and exposed more or less of its body to the water.

But that leaves one critical part of the circuit open - the victim's own tissue. For this measurement, Catania contributed his own arm.

Using a specially-built apparatus that enabled him to collect and measure the electrical current flowing through him, Catania allowed "Finless" (the pet name he has for his test eel) to rear and deliver a volley of 20 or so shocks to his arm sufficient numbers of times to record the data he needed. A shock volley from the eel, his measurements showed, registered about 40-50mA of current at just shy of 200V. The resistance of the arm tissue was about 2000 ohms, which was broadly comparable to the internal resistance of the eel's own body.

This explains why the smaller animals probably jump further out of the water to deliver their shock, which is another observation Catania has made. "It's possible that small eels, with greater internal resistance, must leap to greater heights to divert sufficient current to their target," Catania explains.

This corresponds to a peak power delivery of about 4W from just a small eel. That's about half of the power of a TASER. "That was still sufficient to be quite a deterrent!" says Catania.

But a larger, fully-grown specimen, Catania's measurements suggest, would be capable of delivering over 60W, or almost ten TASERS at once. Ouch!